This invention relates generally to a braking system for use principally on a large vehicle and, more particularly, to an eddy-current type retarder for assisting a friction brake.
In Japanese Patent Laid-open Publication No. 298947/1989, there is disclosed a braking system in which a pair of control plates is rotatably arranged between a pair of brake disks connected to a rotational shaft and a magnet support disk is disposed between the control plates. The magnet support disk supports a plurality of permanent magnets circumferentially spaced at a uniform pitch and with alternating polarities. Retained by the control plates are circumferentially spaced apart ferromagnetic pole pieces having lengths substantially the same as those of the permanent magnets.
When the control plates position each of the pole pieces directly adjacent to one of the permanent magnets, magnetic flux is circulated from the permanent magnets through the ferromagnetic pole pieces to the brake disks. Consequently, an eddy current is generated in the brake disks which receive braking torque.
When the control plates position each of the pole pieces uniformly adjacent to a pair of adjacent permanent magnets a magnetic closed circuit is formed between the adjacent permanent magnets and the pole piece aligned therewith. Consequently, the magnetic field applied to the brake disks becomes extremely small and braking torque on the brake disks is substantially eliminated.
However, in the above braking system a problem is presented by magnetic flux that leaks from the center portions of the permanent magnets, not covered by an adjacent ferromagnetic pole piece. That leakage flux acts on the brake disks producing undesirable dragging torque. If the thickness of the control plates is increased in order to minimize leakage magnetic flux from the permanent magnets to the brake disks, the entire device increases in weight, and spacing between the permanent magnets and brake disks becomes greater. Therefore, magnetic flux circulated by the ferromagnetic pole pieces to the brake disks is reduced and braking torque is diminished.
Disclosed in Japanese Patent Laid-open Publication No. 234044/1989 is another eddy current type braking system in which two permanent magnets of opposite polarity are sandwiched between a pair of ferromagnetic plates during non-braking periods to form short-circuited magnetic closed circuits. In the disclosed system leakage magnetic flux is reduced. However, the arrangement of the ferromagnetic plates and the permanent magnets is irregular with spacing and pitch that are coarse and dense. Consequently, braking torque per unit area received by the brake disks during braking periods is relatively small.
The object of the invention, therefore, is to provide an improved eddy current type retardation device which without increased weight substantially eliminates magnetic leakage flux during non-braking periods and generates high braking torque during braking periods.